2,509 research outputs found
Heat transport in turbulent Rayleigh-Benard convection: Effect of finite top- and bottom-plate conductivity
We describe three apparatus, known as the large, medium, and small apparatus,
used for high-precision measurements of the Nusselt number N as a function of
the Rayleigh number R for cylindrical samples of fluid and present results
illustrating the influence of the finite conductivity of the top and bottom
plates on the heat transport in the fluid. We used water samples at a mean
temperature of 40 degrees C (Prandtl number sigma = 4.4). The samples in the
large apparatus had a diameter D of 49.69 cm and heights L = 116.33, 74.42,
50.61, and 16.52 cm. For the medium apparatus we had D = 24.81 cm, and L =
90.20 and 24.76 cm. The small apparatus contained a sample with D = 9.21 cm,
and L = 9.52 cm. For each aspect ratio Gamma = D/L the data covered a range of
a little over a decade of R. The maximum R = 10^12 with Nusselt numbers N = 600
was reached for Gamma = 0.43. Measurements were made with both Aluminum and
Copper top and bottom plates of nominally identical size and shape. For the
large and medium apparatus the results with Aluminum plates fall below those
obtained with Copper plates, thus confirming qualitatively the prediction by
Verzicco that plates of finite conductivity diminish the heat transport in the
fluid. The Nusselt number N_infinity for plates with infinite conductivity was
estimated by fitting simultaneously Aluminum- and Copper-plate data sets to an
effective powerlaw for N_infinity multiplied by a correction factor f(X) = 1 -
exp[-(aX)^b] that depends on the ratio X of the thermal resistance of the fluid
to that of the plates as suggested by Verzicco. Within their uncertainties the
parameters a and b were independent of Gamma for the large apparatus and showed
a small Gamma-dependence for the medium apparatus. The correction was larger
for the large, smaller for the medium, and negligible for the small apparatus.Comment: 35 pages, 11 figures. Under consideration for publication in Phys. of
Fluid
Competition between Spiral-Defect Chaos and Rolls in Rayleigh-Benard Convection
We present experimental results for pattern formation in Rayleigh-Benard
convection of a fluid with a Prandtl number, Pr~ 4. We find that the
spiral-defect-chaos (SDC) attractor which exists for Pr~1 has become unstable.
Gradually increasing the temperature difference from below to well above its
critical value no longer leads to SDC. A sudden jump of temperature difference
from below to above onset causes convection to grow from thermal fluctuations
and does yield SDC. However, the SDC is a transient; it coarsens and forms a
single cell-filling spiral which then drifts toward the cell wall and
disappears.Comment: 9 pages(RevTeX), 5 jpg figures, To appear as Rapid Communication in
PR
Interplay between the electrical transport properties of GeMn thin films and Ge substrates
We present evidence that electrical transport studies of epitaxial p-type
GeMn thin films fabricated on high resistivity Ge substrates are severely
influenced by parallel conduction through the substrate, related to the large
intrinsic conductivity of Ge due to its small bandgap. Anomalous Hall
measurements and large magneto resistance effects are completely understood by
taking a dominating substrate contribution as well as the measurement geometry
into account. It is shown that substrate conduction persists also for well
conducting, degenerate, p-type thin films, giving rise to an effective
two-layer conduction scheme. Using n-type Ge substrates, parallel conduction
through the substrate can be reduced for the p-type epi-layers, as a
consequence of the emerging pn-interface junction. GeMn thin films fabricated
on these substrates exhibit a negligible magneto resistance effect. Our study
underlines the importance of a thorough characterization and understanding of
possible substrate contributions for electrical transport studies of GeMn thin
films.Comment: 9 pages, 9 figure
Clustering in a precipitate free GeMn magnetic semiconductor
We present the first study relating structural parameters of precipitate free
Ge0.95Mn0.05 films to magnetisation data. Nanometer sized clusters - areas with
increased Mn content on substitutional lattice sites compared to the host
matrix - are detected in transmission electron microscopy (TEM) analysis. The
films show no overall spontaneous magnetisation at all down to 2K. The TEM and
magnetisation results are interpreted in terms of an assembly of
superparamagnetic moments developing in the dense distribution of clusters.
Each cluster individually turns ferromagnetic below an ordering temperature
which depends on its volume and Mn content.Comment: accepted for publication in Phys. Rev. Lett. (2006). High resolution
images ibide
Power-Law Behavior of Power Spectra in Low Prandtl Number Rayleigh-Benard Convection
The origin of the power-law decay measured in the power spectra of low
Prandtl number Rayleigh-Benard convection near the onset of chaos is addressed
using long time numerical simulations of the three-dimensional Boussinesq
equations in cylindrical domains. The power-law is found to arise from
quasi-discontinuous changes in the slope of the time series of the heat
transport associated with the nucleation of dislocation pairs and roll
pinch-off events. For larger frequencies, the power spectra decay exponentially
as expected for time continuous deterministic dynamics.Comment: (10 pages, 6 figures
Heat transport by turbulent Rayleigh-B\'enard convection for $\Pra\ \simeq 0.83\times 10^{12} \alt \Ra\ \alt 10^{15}\Gamma = 0.50$
We report experimental results for heat-transport measurements, in the form
of the Nusselt number \Nu, by turbulent Rayleigh-B\'enard convection in a
cylindrical sample of aspect ratio ( m is
the diameter and m the height). The measurements were made using
sulfur hexafluoride at pressures up to 19 bars as the fluid. They are for the
Rayleigh-number range 3\times 10^{12} \alt \Ra \alt 10^{15} and for Prandtl
numbers \Pra\ between 0.79 and 0.86. For \Ra < \Ra^*_1 \simeq 1.4\times
10^{13} we find \Nu = N_0 \Ra^{\gamma_{eff}} with , consistent with classical turbulent Rayleigh-B\'enard convection in a
system with laminar boundary layers below the top and above the bottom plate.
For \Ra^*_1 < \Ra < \Ra^*_2 (with \Ra^*_2 \simeq 5\times 10^{14})
gradually increases up to . We argue that above
\Ra^*_2 the system is in the ultimate state of convection where the boundary
layers, both thermal and kinetic, are also turbulent. Several previous
measurements for are re-examined and compared with the present
results.Comment: 44 pages, 18 figures, submitted to NJ
Spiral Defect Chaos in Large Aspect Ratio Rayleigh-Benard Convection
We report experiments on convection patterns in a cylindrical cell with a
large aspect ratio. The fluid had a Prandtl number of approximately 1. We
observed a chaotic pattern consisting of many rotating spirals and other
defects in the parameter range where theory predicts that steady straight rolls
should be stable. The correlation length of the pattern decreased rapidly with
increasing control parameter so that the size of a correlated area became much
smaller than the area of the cell. This suggests that the chaotic behavior is
intrinsic to large aspect ratio geometries.Comment: Preprint of experimental paper submitted to Phys. Rev. Lett. May 12
1993. Text is preceeded by many TeX macros. Figures 1 and 2 are rather lon
Minimal Cosmogenic Neutrinos
The observed flux of ultra-high energy (UHE) cosmic rays (CRs) guarantees the
presence of high-energy cosmogenic neutrinos that are produced via
photo-hadronic interactions of CRs propagating through intergalactic space.
This flux of neutrinos doesn't share the many uncertainties associated with the
environment of the yet unknown CR sources. Cosmogenic neutrinos have
nevertheless a strong model dependence associated with the chemical
composition, source distribution or evolution and maximal injection energy of
UHE CRs. We discuss a lower limit on the cosmogenic neutrino spectrum which
depends on the observed UHE CR spectrum and composition and relates directly to
experimentally observable and model-independent quantities. We show explicit
limits for conservative assumptions about the source evolution.Comment: 6 pages, 3 figure
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